More-Specific Prefix
Also known as: more specific route, longest prefix match
A prefix with a longer mask that covers part of a larger block; it always wins in routing over the less-specific covering route.
Routers forward using longest-prefix match: given 10.0.0.0/8 and 10.1.2.0/24 both in the table, traffic to 10.1.2.5 follows the /24 because it is more specific. This is fundamental to how routing works and how address space is subdivided.
More-specifics are a legitimate traffic-engineering and DDoS-mitigation tool, but announcing an unauthorized more-specific of someone else's prefix is also the classic BGP hijack - which is why ROA maxLength and RPKI validation matter.